1. Field of the Invention
The present invention relates to a method of removing a sulfur component in molten iron, and more particularly to an improved desulfurizing method that provides enhanced desulfurization efficiency.
2. Description of the Related Art
As the demand for higher quality steel materials has risen, so also has the demand for low-sulfur steel. The desulfurizing process in steel manufacturing techniques is mainly of two types, i.e., one carried out in the molten iron stage, in a torpedo car or a molten-iron pan, and the other carried out in the molten-steel stage, on deoxidized molten steel downstream of a converter. At present, it is common to carry out the desulfurizing process in both the molten-iron stage and the molten-steel stage, for extremely-low-sulfur steel in which the sulfur content of the molten steel is not larger than 10 ppm, and to carry out the desulfurizing process only in the molten-iron stage for other types of steel.
For the desulfurizing process carried out in the molten-iron stage, a CaO-based desulfurizing agent, an Na2O-based desulfurizing agent, an Mg-based desulfurizing agent, etc. are employed. More specifically, in the desulfurizing process carried out in the molten-iron stage, because the CaO-based desulfurizing agent is preferred from the viewpoints of slag treatment and cost, a technique of improving the efficiency of the process for desulfurizing molten iron by the use of the CaO-based desulfurizing agent is required.
As desulfurizing is a reducing reaction, Japanese Examined Patent Application Publication No. 5-43763 discloses a method of accelerating desulfurization with hydrogen gas. According to this Publication, by employing hydrogen gas as a carrier gas used for blowing a CaO-based desulfurizinq agent, the desulfurizing reaction with the CaO-based desulfurizing agent is accelerated in comparison with the case of employing an inert gas as the carrier gas.
Also, in Japanese Examined Patent Application Publication No. 7-5953, a test is described, as a comparative example, using a hydrocarbon-based gas that also has a reducing property. As a result of the test, it is concluded that the hydrocarbon-based gas is not suitable for the desulfurizing reaction because the temperature of molten iron is lowered due to the endothermic decomposition reaction that occurs upon blowing the hydrocarbon-based gas.
Furthermore, Japanese Examined Patent Application Publication No. 63-19562 discloses a method of accelerating the desulfurizing reaction by adding a desulfurizing agent to molten iron from above and blowing a hydrocarbon-based gas thereto from below in a molten-iron trough of a blast furnace. Moreover, Japanese Unexamined Patent Application Publication No. 60-26607 discloses a method of mixing, in a CaO-based desulfurizing agent, an organic material that contains 3-20 weight % of coal-based hydrocarbon.
As noted above, when a hydrocarbon-based gas is blown into molten iron, the temperature of molten iron is lowered due to the endothermic decomposition reaction of the hydrocarbon-based gas. However, we have recognized that the efficiency of the desulfurizing reaction is improved by blowing the hydrocarbon-based gas into molten iron while holding the temperature of the molten iron at a high level. In the case of blowing the hydrocarbon-based gas into the molten iron, therefore, we have recognized that it is required to limit the amount of the hydrocarbon-based gas used in an appropriate range.
Furthermore, we have recognized that if the position at which a desulfurizing agent is blown in differs from the position at which a hydrocarbon-based gas is blown in, the desulfurizing agent and the hydrocarbon-based gas do not mix with each other sufficiently, thus resulting in reduced efficiency of the desulfurizing reaction. The method of employing, as a desulfurizing agent, an organic material containing coal-based hydrocarbon has the problems that the production cost is pushed up by an expensive cost of such an organic material, and the coal-based hydrocarbon cannot be selectively supplied depending on a sulfur concentration level of the molten iron.
With the view of solving the problems set forth above, an object of the present invention is to provide a desulfurizing method which, when the desulfurizing process is carried out by blowing a CaO-based desulfurizing agent into molten iron, can improve the desulfurization efficiency of the CaO-based desulfurizing agent, can increase the productivity of the desulfurizing process, and can reduce the amount of slag generated in the desulfurizing process.
Generally, the desulfurizing reaction of molten iron effected by a CaO-based desulfurizing agent is expressed by the following formula (1). In the formula (1), [S] denotes S (sulfur) in the molten iron. Also, [C] denotes C (carbon) in the molten iron and contributes, as a reductant, to the desulfurizing reaction in the formula (1). Further, (CaS) indicates that CaS is removed with slag.
[S]+CaO+[C]xe2x86x92(CaS)+COxe2x80x83xe2x80x83(1)
When a hydrocarbon-based gas, which is a reducing gas, is blown into molten iron, the hydrocarbon-based gas is decomposed to produce a hydrogen gas. This reaction is expressed by the following formula (2):
CnHmxe2x86x92nC+m/2H2xe2x80x83xe2x80x83(2)
The desulfurizing reaction effected by the hydrogen gas and the CaO-based desulfurizing agent occurs as expressed in the following formula (3). The desulfurizing reaction of the formula (3) is more advantageous because of having higher reducing power than the reducing reaction effected by C in the molten iron. Considering the case where hydrocarbon (e.g., propane) contributes directly to the reaction, that case is expressed by the following formula (4), i.e., the sum of the two formulae (3) and (2) (n=3 and m=8). Thus, comparing both the desulfurizing reactions (3) and (4) in terms of free energy of the reaction, the desulfurizing reaction (4) based on hydrocarbon is more advantageous than the desulfurizing reaction (3) based on hydrogen by an amount corresponding to decomposing reaction energy of the hydrocarbon.
[S]+CaO+H2xe2x86x92(CaS)+H2Oxe2x80x83xe2x80x83(3)
7[S]+7CaO+C3H8xe2x86x927(CaS)+4H2O+3COxe2x80x83xe2x80x83(4)
However, because decomposition of the hydrocarbon-based gas, shown in the above formula (2), occurs as an endothermic reaction, blowing of the hydrocarbon-based gas causes a reduction in the temperature of the molten iron. In other words, blowing a large amount of the hydrocarbon-based gas reduces the temperature of the molten iron and impedes the desulfurizing reaction. It is therefore required to limit the amount of the hydrocarbon-based gas used in an appropriate range.
In view of the above, the inventors have attained new findings and-accomplished the present invention as follows.
(1) The present invention resides in a method of desulfurizing molten iron by blowing a powdery desulfurizing agent, which contains a solid oxide as a main component, into the molten iron together with a carrier gas, the method comprising the steps of using, as the carrier gas, a gas mixture of an inert gas and a hydrocarbon-based gas; and setting a ratio of the hydrocarbon-based gas to the desulfurizing agent to be in the range of 2.0 to 50 Nl/kg.
(2) In the above method of desulfurizing molten iron, preferably, a blowing rate of the desulfurizing agent is not greater than 1.0 kg/minute per ton of the molten iron.
(3) Also, the present invention resides in a method of desulfurizing molten iron by blowing a desulfurizing flux into the molten iron together with a carrier gas and removing sulfur in the molten iron, the method comprising the steps of using, as the carrier gas, a gas mixture of an inert gas and a hydrocarbon-based gas at the start of desulfurization; and increasing a proportion of the hydrocarbon-based gas in the carrier gas or replacing the carrier gas by the hydrocarbon-based gas when a sulfur concentration in the molten iron is reduced down to or below a predetermined value after the start of desulfurization.
(4) Furthermore, the present invention resides in a method of desulfurizing molten iron by blowing a desulfurizing flux into the molten iron together with a carrier gas and removing sulfur in the molten iron, the method comprising the steps of using an inert gas as the carrier gas at the start of desulfurization; and adding a hydrocarbon-based gas to the carrier gas or replacing the carrier gas by a hydrocarbon-based gas when a sulfur concentration in the molten iron is reduced down to or below a predetermined value after the start of desulfurization.
In the above method, it was found that the predetermined value of the sulfur concentration is preferably set to 0.01 wt %.